Exhaust gas aftertreatment systems for reducing NOx emissions of internal combustion engines are known. With such systems, it is desirable to determine at any given time a NOx conversion efficiency of one or more NOx-reducing catalysts forming part of the exhaust gas aftertreatment system. It may be desirable, for example, to then control the NOx output of the engine as a function of the NOx conversion efficiency of the one or more NOx-reducing catalysts.